You can't change how someone feels about something, no matter how shit it is. You will never be able to change his mind- only he can do that.
However, what you can do is create a calm, loving environment for you, baby and your children. With all due respect- he's being pathetic. He has two children, he knows what to do. His life isn't "over" and I'd be confident in saying that he's only hyped up these feelings to try and get you to do what he wants. You didn't, so now he is punishing you for it. This makes him nothing more than a bully.
He doesn't get to punish you for something he had a 50% role in. Stop letting him punish you. Stop punishing yourself. Focus entirely on your children and your baby.
If it was me, I'd explain to him in very clear terms that if his attitude doesn't swiftly change, he'd be swiftly out on his ear. I'd explain that if he didn't want a third child, he should have taken contraceptive measures into his own hands, and not fully expected another human being to go through something traumatic just so he wouldn't have a few more years of changing nappies. Make it clear that his feelings would no longer register or impact on you. He didn't care about your feelings, remember. He wanted you to go something that would destroy you, purely for his benefit. So fuck his feelings.
As I said, you can't force him to change. You also can't force him to be a good dad to this baby. You've made the decision to keep the baby, so as a result, you need to prepare to be a single mum. You might as well bin him off now if he isn't going to step up, enjoy the pregnancy and create a stress-free environment for your family, rather than sticking with him where he'll grind you into the ground every day, and then be a shit dad as well.